Field of the Invention
The present invention relates to a radiation imaging apparatus and a method for controlling the same.
Description of the Related Art
Currently, in X-ray still image imaging systems in medicine, film methods are common in which X-rays are irradiated onto an object to be imaged, and the transmitted X-ray image is exposed on a film. Film methods offer the functionality of displaying and recording information, they permit for a large area, are excellent in gradation characteristics, and in addition, are light weight and easy to handle. Thus, film methods are widespread throughout the world. However, film methods require space, man-power and time, due to the complication of requiring a development process, as well as for long-term storage and accessibility.
Recently, the demand for digital X-ray images has been increasing in hospitals. Radiation imaging apparatuses that, instead of films, convert radiation into electric signals by a plurality of radiation detection elements arrayed in a two-dimensional matrix and form an image have been put to use. In these types of radiation imaging apparatuses, an X-ray detector (FPD: Flat Panel Detector) in which solid-state imaging elements are arrayed in a two-dimensional matrix, and which converts an X-ray dose into an electric signal is used. With X-ray imaging apparatuses having such an X-ray detector, since it is possible to convert an X-ray image into digital information, it is possible to transmit image information to a remote location, and, what is more, instantaneously. By transmitting information of an X-ray image, there is the advantage that it is possible to receive an advanced diagnosis equal to that of a university hospital in a city center while in a remote location. In addition, films are not used, and thereby, there is the advantage that storage space required for films in hospitals is reduced. In the future, with the introduction of superior image processing techniques, the realization of automated diagnoses in which a diagnosis is made using a computer without a radiologist is expected.
X-ray detectors such as FPDs (hereinafter, referred to as sensors) include a photoelectric conversion circuit in which a plurality of photoelectric conversion elements that convert radiation into an electric signal are arrayed in a matrix, and a read-out circuit for reading out, from this photoelectric conversion circuit, the electric signal obtained by this conversion. When X-rays are irradiated onto an object to be imaged, the photoelectric conversion according to the transmitted X-rays is performed by photoelectric conversion elements of a photoelectric conversion circuit, and the signal charges corresponding to the transmitted X-ray doses are accumulated in the photoelectric elements. A read-out circuit drives the signal lines of the photoelectric conversion circuit, and controls, as appropriate, switching elements to which the photoelectric conversion elements are connected. Thereby the read-out circuit successively reads out the signal charges accumulated in the photoelectric conversion elements as electric signals, amplifies them, and outputs them.
By the operation as described above, it is possible to read an image of the person to be imaged with the electric signals output from the read-out circuit. However, an offset generated in the photoelectric conversion circuit and the read-out circuit is included in the image thus read-out (the electric signals output from the read-out circuit, representing the image). Causes of the offset may be: (A) dark currents in the photoelectric conversion elements, (B) leak currents in the switching elements, (C) offset voltages in the amplifiers of the read-out circuit, and the like.
Since images obtained by irradiating X-rays include offsets as described above, the offset components must be removed. The processing for removing such an offset component is referred to as offset correction. The processing of ordinary offset correction will now be described. FIG. 7 is a diagram showing a timing chart of ordinary offset correction. When a still image is imaged, if an X-ray exposure is performed at the timing shown in chart TC701 by an exposure instruction of an operator, the X-ray imaging apparatus drives a sensor at the timing shown in chart TC702, and obtains an X-ray image (X) (at timing 711 in FIG. 7). Subsequently, the X-ray imaging apparatus drives the sensor in a state in which no X-rays are irradiated, and obtains an offset image (F) (timing 712 in FIG. 7). The X-ray imaging apparatus subtracts the above-described offset image (F) from the above-described X-ray image (X) at the timing shown in chart TC703 (X-F), and displays a preview image for the operator to determine the success/failure of the imaging, at the timing shown in chart TC705. Furthermore, the X-ray imaging apparatus performs an image analysis for using the image (X-F) in an actual diagnosis, at the timing shown in chart TC704, by using the image (X-F) in which offset correction is performed. When this is finished, a diagnostic image is displayed at the timing shown in a chart TC706.
However, in the above-described offset correction method, since the offset image is obtained after X-ray irradiation (timing 712), there is a problem in that the time period (T6 in FIG. 7) from the X-ray irradiation to the display of a preview image becomes longer. In order to shorten the time period from X-ray irradiation to the display of a preview image, a method for obtaining an offset image before X-ray irradiation, the imaging by a video camera or the like is described in Japanese Patent No. 03190328 (hereinafter, Document 1). In the method described in Document 1, offset images are periodically obtained, and the offset image obtained at the timing closest to imaging is used for the actual correction.
However, if offset images are periodically obtained as in Document 1, when the timing for obtaining an offset image and the instruction of an X-ray exposure overlap, the X-ray exposure is performed after obtaining the offset image. That is, there are cases where variations occur in the time periods from the instruction of an X-ray exposure to the X-ray exposure, and the instantaneousness of the X-ray exposure instructions is compromised. In addition, in the analysis of an X-ray image, although it is preferable to use the offset image immediately after the X-ray image is obtained, an offset image before an X-ray image is obtained is used in the technique in Document 1. Therefore, it is difficult to assure the accuracy of the offset correction. Particularly, high-accuracy offset correction is difficult at or near the startup of an apparatus in which the amount of offset fluctuates.